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Anal. Chem. 1989, 6 1 , 1431-1435
Light Absorption and Mixed Micelle Composition as Factors in Determining Intensities of Room-Temperature Phosphorescence N. E. N u g a r a a n d A. D. King, Jr.* Department of Chemistry, School of Chemical Sciences, University of Georgia, Athens, Georgia 30602
A series of experlments have been performed that demonstrate (a) that problems arlslng from precipitation of TI+-alkyl sulfate salts at high TI’ concentratlons In mlcelle-stabilized room-temperature phosphorescence (MS-RTP) solutions can be avoided by using mlxed surfactant systems that include a short chaln alkyl sulfate and (b) that In cases where the sulfite Ion Is used as a chemlcal deoxygenating agent, light absorption by a TI’-SOt- complex can severely attenuate the excltlng radiatlon, particularly at wavelengths below 275 nm, thus offsetting any gains derived from lncreaslng the TI’ content of MS-RTP soiutlons through the use of mixed surfactants. The stoichlometry, formation constant, and molar absorptivity of this Ti+-SO:complex are determined by uslng UV spectrometry. Fluorescence and thallium-lnduced phosphorescence Intensity data derived from three aromatlc compounds solubilized In a mixed micelle MS-RTP system containing Na,SO, are Included to Illustrate the advantages of long Wavelength excltation in such MS-RTP systems.
INTRODUCTION Micelle-stabilized room-temperature phosphorescence (MS-RTP) is an interesting manifestation of the external heavy-atom effect, which shows great promise as a means of detection in liquid chromatography (1-16). Basically, MSRTP achieves its effect by utilizing the amphiphilic character of micellized surfactant ions to compartmentalize solubilized lumiphore molecules, normally aromatics, in a submicroscopic region surrounded by a high concentration of heavy-atom counterions. This facilitates collisions between the two species and has the effect of increasing the flux of energy passing through the phosphorescence channel by increasing triplet quantum yields and perhaps phosphorescence efficiencies as well. Anionic micelles composed of dodecyl sulfate anions surrounded by a counterion sheath containing either T1+or Ag+ cations are commonly used to effect MS-RTP although micelles of the cationic type composed of cetyltrimethylammonium bromide have been shown to induce MS-RTP as well (11). While lumiphore concentration effects attributed to triplet-triplet annihilation and the inner-filter effect ( 5 ) , as well as a specificity of Ag+ relative to T1+ toward solubilized heterocyclic compounds (15) have been found to affect phosphorescence intensities in MS-RTP, there are two fundamental conditions that must be met in order to maximize the intensity of MS-RTP: (a) the relative concentration of heavy-atom ions relative to other counterions in the micellar solution must be as high as possible and (b) the concentration of 0 2 ,a potent triplet quencher, must be minimized. In practice, however, the low solubilities of heavy-metal dodecyl sulfate salts at room temperature place a ceiling (approximately 30 mol % in the case of T1+) on the amount of heavy-metal ions that can be introduced into the commonly used MS-RTP systems based on sodium dodecyl sulfate. Likewise, the traditional method of using N2 purging to re0003-2700/89/0361-1431$01.50/0
move O2from solution has been of limited utility because of foam generation and its concomitant problems. Because of this, the recent discovery that sulfite ions act as efficient O2 scavengers in micellar solutions used for MS-RTP by Garcia and Sanz-Medel(16) represent a major technical advance in the use of MS-RTP. The purpose of this paper is 2-fold. First, it will be shown that for MS-RTP systems that employ T1+, the solubility limitations encountered with sodium dodecyl sulfate (SDS) can be greatly ameliorated through the use of a mixed surfactant system composed of SDS blended with a shorter chain surfactant, sodium octyl sulfate (SOS). Secondly, evidence will be presented to show that light absorption by a 1:l comacts to diminish the intensity of plex of Tlf with SO-: phosphorescence generated by micellar solutions which use T1+ as a heavy atom perturber and S032-as a chemical deoxygenating agent. EXPERIMENTAL SECTION Instrumentation. The UV absorption spectra of the micellar solutions of interest were obtained by using a Hewlett-Packard Model 8451-A diode array spectrophotometer. The emission spectra were measured with a Perkin-Elmer Model MPF-44B spectrofluorometer,equipped with a differential corrected spectral unit (DSCU) and a thermostated cell compartment that utilizes a Brinkmann-Lauda super K-2/R circulating constant temperature bath to control the temperature to 25 0.5 “C for the work reported here. Surface tension measurements used to determine values for the critical micelle concentration (cmc) of purified SDS, SOS, and their mixtures were obtained by using a Fisher Autotensiomat, Model 215, automated duNouy tensiometer having a Haake Model FS constant temperature circulator to maintain solution temperature to 0.1 “C. Chemicals and Reagents. The SDS and SOS used in this work were purchased from BDH Chemicals Ltd. (lot no. 9088113 C 500) and Eastman Kodak Co. (lot no. 16A), respectively. Each was recrystallized from 1:l (for SDS) and 1:3 (for SOS) (v/v) ethanol/2-propanolmixtures and dried in vacuo prior to use. The purity of each surfactant with respect to alkyl group chain length was verified by comparing cmc values obtained with aqueous solutions of the recrystallized SOS and SDS with recommended literature values (17). Good agreement was found in each case. Thallium(1) nitrate, purchased from Fluka Chemie A.G., was recrystallized twice from H 2 0 and dried in vacuo. Sodium sulfite (anhydrous),purchased from J. T. Baker Chemical Co., was used as received. The naphthalene, Baker Analyzed Reagent Grade, purchased from J. T. Baker Chemical Co., was used as received while the phenanthrene and pyrene, both Eastman White Label Grade, were purchased from Eastman Kodak Co. and were recrystallized from ethanol prior to use. Doubly distilled water was used to prepare all solutions. The luminescence intensities shown in Figures 1 and 4 are “uncorrected spectra” with no corrections being made for the wavelength dependence of instrument response. The 15-nm slits were used for both excitation and emission.
*
*
R E S U L T S AND DISCUSSION Concentration-temperature phase diagrams for aqueous systems containing ionic surfactants have one feature in 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61,NO. 13,JULY 1, 1989
common, namely that the solid solubility-temperature curves exhibit an abrupt increase in slope at a temperature known as the Krafft temperature, TK (18). Short-chain surfactants have low Krafft temperatures and hence tend to be very soluble and readily form micellar solutions at ordinary temperatures. Long-chain surfactants on the other hand are characterized by high Krafft temperature and become quite insoluble whenever TKexceeds ambient temperature. Mixtures of short- and long-chain surfactants have Krafft temperatures that are intermediate to those of the two pure species. Large polarizable counterions, such as T1+ in the case of SDS, have the effect of increasing TK and hence reducing solubility. This effect is roughly proportional to the concentration of the large counterions and the addition of a salt containing such an ion will cause a surfactant to precipitate whenever the concentration of that counterion reaches a level sufficient to raise TKabove ambient temperature. As noted earlier, in the case of SDS, precipitation occurs when the T1+ content reaches approximately 30 mol % relative to Na+, i.e.
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chain and a long-chain surfactant of the same class will be more resistant to precipitation by polarizable counterions than the long-chain surfactant alone. This is found to be the case with MS-RTP systems based on SDS and Tl'. Here one finds that the increased resistance to precipitation afforded by the addition of sodium octyl sulfate to make a 20 mol % SOS-80 mol % SDS blend allows one to obtain spectra with T1+ contents reaching (Tl+/Na+)% = 50% without precipitation. Figure 1 is derived from intensities of fluorescence, If,and phosphorescence, Zp, measured a t 325 nm and 476 nm, respectively, for naphthalene solubilized in three different micellar solutions composed of SDS, a SDS/SOS blend (80% SDS), and pure SOS, each containing 0.005 M Na2S03and varying amounts of T1N03. The absorbance of naphthalene dissolved in each solution is 1.00 at the wavelength used for excitation, A,, = 276 nm. The cmc values measured in the absence of added salt a t 25 O C for SDS, the 80% SDS/SOS M, 9.0 X M, and 0.13 blend, and pure SOS are 8.0 X M, respectively. Therefore, to a first approximation, each of these solutions contains the same concentration (0.1 M) of micellized alkyl sulfate ions. Figure l a shows the phosphorescence intensity measured with each of these solutions plotted as a function of T1+ content expressed as (Tl+/Na+)%. While the intensities obtained by using the SDS/SOS blend do not differ appreciably from those obtained with pure SDS at low T1+ concentrations, one sees that Ipfalls off sharply at higher concentrations so that little is gained by using the mixed surfactant system to achieve T1+concentrations in excess of the precipitation limit for SDS, i.e. greater than (Tl+/Na+)% = 30%. The phosphorescence intensities observed with pure SOS under the same conditions are less intense and pass through a similar though less pronounced maximum a t a thallium content of (Tl+/Na+)% = 30%. The errors associated with the intensities of phosphorescence, I,, and fluorescence, I f , originate from instrumental error, uncertainties in thallium ion and sodium ion concentrations (hence (Tl+/Na+)%),as well as errors in the concentrations of the solubilized aromatic lumiphore, sulfite ion, and residual 02. The sensitivity of Ip(or If)to each of these factors is unknown although each of the chemically related sources of error is expected to become more severe at shorter excitation wavelengths. However, an error estimate based on replicate spectra obtained with naphthalene at two different
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(Tl+/Na+)Olo Flgure 1. (a) Intensity (arbPrary units) of phosphorescence, I,, at 476 nm for naphthalene solubilized in 0.10 M SDS (O), 0.23 M SOS (A), and 0.080M SDSIO.020 M SOS (0),each with 0.005M Na,SO, and varying TINO, concentrations at 25 OC, shown as a function of (TI+/Na+)%. A = 276 nm. [Naphthalene] = 1.67X lo-' M. (b) Tha ratio of inte&es, (Ip/l,), for naphthalene solubiliied in the micellar solutions used in Figure l a , as denoted by the same symbols, plotted as a function of (TI+/Na+)%.
(Tl+/Na+)%using 276 nm excitation indicates that Zp and Ifcan be reproduced to within * 8 % using a MS-RTP system based on a 0.10 M 20% SOS-80% SDS mixture. Since Ipand If used to calculate individual values of I p / I f are measured in the same experiment, only instrumental error, uncertainties in (Tl+/Na+)% and residual O2concentration contribute to the error associated with this ratio and the error limit for the ratio Zp/If based on this same set of experiments is found to be *12%. Two factors may be responsible for this fall off in phosphorescence intensity at high T1+ion concentrations. First, T1+ ions concentrated about the alkyl sulfate micelles may act as quenching agents by increasing the rate of nonradiative energy loss from the lowest triplet state of the solubilized naphthalene molecules. Alternatively, the T1+ions may cause a reduction in luminescence by simply reducing the intensity of light at the excitation frequency that reaches the solubilized naphthalene, presumably by forming a light absorbing complex with one or more of the species in solution. The ratio of phosphorescence and fluorescence intensities, IP/Zf, obtained with uncorrected emission spectra having constant band shapes can be shown to be a function solely of the various rate constants that govern the internal channels for energy flow within the lumiphore; i.e.
IP/If= (const)-
kp:rkGT}
Here km represents the rate constant for intersystem crossing between the lowest excited singlet (SI)and triplet (TI)states. The term kGT is the rate constant governing nonradiative
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13,JULY 1, 1989
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WAVELENGTH (nm) Figure 2. Absorption spectra of naphthalene solubilized in 0.23 M SOS and 0.019 M TINO, (a) before and (b) after the addition of 0.005 M Na,SO, at 25 OC: path length, 1 cm; [naphthalene] = 1.67 X lo-' M.
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decay of T1while kFM and kpT are rate constants governing the radiative decay of S1 and T1,respectively. Therefore, the fact that the experimentally determined ratios of Ip/Iffor naphthalene in the three different micellar solutions, shown plotted in Figure Ib, all follow a smooth, concave upward path over the full range of (Tl+/Na+)% concentrations available to each system indicates that the loss of phosphorescence intensity observed in Figure l a is not due to a quenching process whereby T1+accelerates the rate of nonradiative decay from T1by increasing kGT but rather is caused by attenuation of the exciting radiation by either light scattering or light absorption by some complex formed by T1+ in solution. The absorption spectra shown in Figure 2 taken with a T1+ based MS-RTP solution containing naphthalene solubilized in SOS support this conclusion. Here it is seen that a broad absorption band develops under the absorption band of naphthalene when Na2S03is added to the solution. This strongly suggests that a thallium(1)-sulfite complex is responsible for the light absorption, although the possibility exists that light scattering by large micelles produced by added sodium sulfite may be the cause of the observed attenuation. Figure 3 compares the absorption spectra obtained with aqueous solutions containing (a) 0.010 M TlN03, (b) 0.010 M Na2S03,and (c) a 1:l mixture of these two solutions, all in the absence of any surfactant. Here it is seen that a strong structureless absorption band develops in the region between 250 nm and the weak NO; band at 310 nm when the two salts are mixed. Since no surfactant is present in these solutions, one concludes that a Tl+-SO?- complex is responsible for the light absorption observed with the mixture in Figure 3 and, by inference, for the light absorption found with the MS-RTP solution shown in Figure 2 as well. A series of spectral measurements were performed to determine the absorbance of binary solutions having various concentrations of TlN03 and Na2S03in order to characterize the Tl+-S032- complex responsible for this light absorption. The first of these entailed using a method developed by Vosburgh and Cooper (19) in which a Job plot is used to determine the stoichiometry of the light-absorbing complex. Here equimolar solutions of TlN03 and Na2S03are mixed in different proportions to produce a series of solutions having the same total volume but different relative concentrations of the two salts. The absorbance values measured for these mixtures, when plotted against the relative concentration of one salt compared to the other, expressed as mole fraction, will yield a curve having negative curvature whose maximum corresponds to the mole fraction representing the stoichiometry of the light-absorbing complex. The upper-left inset of Figure 3 shows the results obtained with T1N03-Na2S03
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WAVELENGTH (nm) Figure 3. Absorption spectra of aqueous solutions containing (a) 0.010 M Na,SO,, (b) 0.010 M TINO,, and (c) a 1:l mixture of 0.010M TINO, and 0.010 M Na,SO,. Path length, 1 cm. Upper left inset: Job plot showing absorbance values plotted as a function of X,t,so,2- for solutions containing different proportions of TINO, and Na2S03at 25 OC. Upper curve, absorbance at 260 nm; lower curve, absorbance at 270 nm. Total concentration of TI' and S O : + is 0.02 M. Optical path length, 1 cm. Upper right inset: Absorptivity of the TISO,- complex in aqueous solution shown as a function of wavelength. (0)Absorptivities obtained directly from nonlinear least-squares fit of absorbance data taken over range 276-292 nm. (0) Absorptivities calculated by using the K,, value obtained from nonlinear least-squares fit of 276-292 nm data with more dilute solutions of TINO, and Na,SO, having differing relative concentrations.
mixtures at two different wavelengths, 260 and 270 nm. Both maxima occur a t a mole fraction of 0.5 indicating that a 1:l complex between T1+ and S032-is responsible for light absorption over the range 260-290 nm seen in Figures 2 and 3. Once the stoichiometry is known, one can utilize similar spectral data to determine the value of the stability constant, K,, for this T1+-S032-complex using a method developed by Yoneda (20). Here, absorbance values were measured at five different wavelengths over the range 276-292 nm for a series of solutions containing 0.0100 M TlN03 and varying amounts of Na2S03. The results obtained a t each wavelength were fitted to the equations derived by Yoneda using nonlinear least-squares methods to yield values for the absorptivity of the complex and K,,. The values obtained for K,, show no trend with wavelength and yield an average value for K,, a t 25 "C [TlSO3-] Ksc
= [T1+][S032-]
=48&3
It is interesting to note that this value is quite similar to that found for the 1:l complex of Tl(1)with sulfate ion which is K,, = 28 a t 25 "C (21). Once the stoichiometry and formation constant are known, it becomes a simple matter to calculate values for the absorptivity of this Tl+-S032-complex using the absorption spectra obtained with these solutions. The results calculated at 4-nm intervals are shown plotted as a function of wavelength in the upper-right inset of Figure 3 along with the values derived by the original least-squares fit. It is clear from these results that the absorbance due to this Tl+-S032- complex can become quite large in MS-RTP solutions using
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ANALYTICAL CHEMISTRY, VOL. 61,NO. 13,JULY 1, 1989 80 70 60
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(TI+/Na+)"lo Flgure 4. (a) Intensity (arbitrary units) of phosphorescence, I p l at 470 nm obtained by using two different excltatlon wavelengths, for phenanthrene solubHized in solutions containing 0.080M SDS, 0.020 M SOS. 0.005 M Na2S0 and varying amounts of TINO:, at 25 O C , shown as a function of (TI%/Na+)%. (0): A,, = 276 nm, (0): A,, = 296 nm. [Phenanthrene] = 5.3 X lo-' M ca. Absorbance (276 nm) = 1.0, 0.84, 1 cm path length. (b) The ratio of absorbance (298 nm) intensltieq (IdI,),for phenanthrene sdubwired in the micellar sdulions used in Figure 4a, as denoted by the same symbols, plotted as a function of (TI+/Na+) % .
Na.,$303 for deoxygenation, particularly at higher thallium concentmtions. For example, ignoring ionic strength effects, one can use the experimentally determined value K, = 48 and the absorptivity values shown in Figure 3 to estimate that the absorbance over a 1-cm path length due to the Tl+-SO;complex in the SOS-SDS MS-RTP solution used in Figure 1will exceed unity at wavelengths less than 276 nm whenever the thallium ion content exceeds (Tl+/Na+)% = 30%. This situation, of course, becomes progressively worse at higher thallium concentrations. The fact that the intensity of phosphorescence depends upon the relative concentrations of T1+ and Na+ in these solutions while the concentration of light-absorbing complex depends upon the concentration of T1+ alone explains why the intensity of phosphorescence of naphthalene solubilized in SOS shown in Figure 1 is weak in comparison with that generated by the solutions containing SDS or the SOS-SDS mixture. The cmc of SOS is 0.13 M at 25 "C (17). Therefore, a surfactant concentration of 0.23 M is required to produce 0.1 mol of micellized surfactant ions per liter, the amount of micellized surfactant ions common to all the data shown in Figure 1. Including the Na2S03,the sodium ion concentration in these SOS solutions is 0.24 M, roughly twice that in the solutions containing SDS or SOS mixed with SDS,both of which are 0.1 M with respect to surfactant. It follows that twice as much T1+ is required to produce a given (Tl+/Na+)% in the SOS solutions, which in turn produces increased con-
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Flgure 5. (a) Intensity (arbitrary units) of phosphorescence, Ip, at 575 nm obtained by using two different excitation wavelengths, for pyrene solubillred in solutions containing 0.080M SDS, 0.020M SOS,0.005 M Na2S03, and va ing amounts of TINO:, at 25 "C, shown as a function of (TI+/Na )%: (0)A, = 276 nm, (0) A,, = 338 nm; [pyrene] = 2.2 X lod M. Absorbance (276 nm) = 1.0,absorbance (338nm) = 1.1, 1 cm path length. (b) The ratio of intensities (Ip/If), for pyrene solubilized in the micellar solutions used in Figure 5a, as denoted by same symbols, plotted as a function of (TI+/Na+)%.
r
centrations of the light-absorbing Tl+-S032- complex. One calculates that the 1cm path length absorbance of the MSRTP solution containing 0.23 M SOS used in Figure 1 will exceed unity whenever the thallium content exceeds (T1+/ Na+)% = 15%. Finally, the absorptivity of the Tl+-SO:complex shown in Figure 3 is seen to fall off rapidly with wavelength. This suggests that, where a choice is possible, much is to be gained by using long wavelength excitation. This is illustrated by phosphorescence data obtained with phenanthrene. The vibronic structure of the 'Laabsorption band of phenanthrene contains two well-defined maxima having nearly the same absorptivity at 276 and 296 nm, respectively. Inspection of the absorptivities of the Tl+-SO:complex shown in Figure 3 leads one to conclude that excitation at 276 nm will be attenuated to a much greater extent than that at 296 nm in MS-RTP solutions thus leading to reduced excitation and hence weaker phosphorescence when 276-nm excitation is used. The data in Figure 4 show this to be the case. A second and more exaggerated example illustrating this same effect is provided by MS-RTP spectra derived from solubilized pyrene shown in Figure 5a. Here, the combined effects of a low triplet quantum yield (kFM is large relative to kTM for the unperturbed molecule) and the high absorptivity of the Tl+-S032- complex at 276 nm render excitation at 276 nm virtually ineffective. On the other hand, illumination at 338 nm, where no attenuation is expected, generates intense phosphorescence from the same solution. The pyrene ab-
Anal. Chem. 1989. 61. 1435-1441
sorption bands a t 276 and 338 nm have nearly identical absorptivities but are derived from optical transitions to two entirely different excited electronic states. Although the experimental error associated with the extremely weak fluorescence and phosphorescence emissions obtained by using 276 nm excitation are quite large, the near identity of the I p l I f ratios shown in Figure 5b indicate that, as expected, rapid interconversion between the excited singlet states of pyrene prevails leaving the phosphorescence quantum yield insensitive to the fact that the excited singlet states resulting from excitation at 276 and 338 nm are different. LITERATURE CITED Kalyanasundram, K.; Grieser, F.; Thomas, J. K. Chem. fhys. Lett. 1977, 57, 501-505. Humphry-Baker, R.; Moroi. Y.; Gratzei, M. Chem. fhys. Lett. 1978, 58,207-210. Turro, N. J.; Llu, K.-C.; Chow, M.-F.; Lee, P. fhotochem. fhotobiol. 1978, 27, 523-529. Almgren, M.; Grieser, F.; Thomas, J. K. J . Am. Chem. SOC. 1979, 10 7 , 279-291. Cline Love, L. J.; Skriiec, M.; Habarta, J. G. Anal. Chem. 1980, 52, 754-759. Skrllec, M.; Cline Love, L. J. Anal. Chem. 1980, 52, 1559-1564. Armstrong, D. W.; Hinze, W. L.; Bul, K. H.; Slngh, H. N. Anal. Lett. 1981, 14(A19), 1659-1667. Skrllec, M.; Cline Love, L. J. J . fhys. Chem. 1981, 8 5 , 2047-2050.
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Cline Love, L. J.; Habarta, J. G.; Skrilec, M. Anal. Chem. 1981, 53, 437-444. Cline Love, L. J.; Skrllec, M. Anal. Chem. 1981, 53,1872-1875. Wolff, T. Bar. Bunsen-Ges. Phys. Chem. 1982, 86, 1132-1134. Weinberger, R.;Yarmchuk, P.; Cline Love, L. J. Anal. Chem. 1982, 5 4 , 1552-1558. Cline Love, L. J.; Weinberger, R., Spectrochim. Acta, Part B 1983. 3 8 8 . 1421-1433. Femla, R. A.; Cline Love, L. J. Anal. Chem. 1984, 56, 327-331. Woods, R.: Cline Love, L. J. Spectrochim. Acta, Part A 1984, 4 0 A , 643-650. Garcia, M. E. D.; Sanz-Medel, A. Anal. Chem. 1986, 58, 1436-1440. Mukerjee, P.;Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems. Natl. Stand. Ref. Data Series ( U . S . ,Natl. Bur. Stand.) 1961, NSRD S-NBS 36. Adamson, A. W. fhysical Chemistry of Surfaces, 4th ed.; John Wiley & Sons: New York, 1982; Chapter X I I I . Vosburgh. W. C.; Cooper, G. R. J . Am. Chem. SOC. 1941, 63, 437-442. Yoneda, H. Bull. Chem. SOC. Jpn. 1956, 29, 68-71. Sillen, L. G.; Marteli, A. E. Stabiltty Constants of Metal Ion Complexes, 2nd ed.; The Chemical Society of London: London, 1964; p 244.
RECEIVED for review December 21, 1988. Accepted March 27, 1989. The results reported here are taken from the M.S. Thesis of N. E. Nugara (University of Georgia, 1988). The authors express appreciation for support provided by the National Science Foundation (NSF) under Grant CHE8218288.
Aromatic Bases as Eluent Components for Conductivity and Indirect Ultraviolet Absorption Detection of Inorganic Cations in Nonsuppressed Ion Chromatography Paul R. Haddad* and Roy C. Foley
Department of Analytical Chemistry, University of New South Wales, P.O. Box 1, Kensington, New South Wales 2033, Australia
A range of protonated aromatlc bases was Investigated as eluents for the nonsuppressed Ion chromatography of lnorganlc cations, uslng simultaneous dlrect conductlvlty and Indirect UV absorption detectlon. When a low-capaclty styrene4lvlnylbenzene cation-exchange column was used, m e thylpyrldlne Isomers, dlmethylpyridlne Isomers, benzylamlne, 2ghenylethylamlne, and 4-methylbenrylamlne proved suitable for the separatlon of alkall-metal cations and ammonium. Detectlon llmits were In the range 0.3-6.7 ppb for conductlvlty detection and 0.2-21.0 ppb for UV absorptlon detection. Alkaline-earth-metal catlons could be separated by using hlgher concentrations of the same eluent specles, glvlng detectlon limits of 9-917 and 1.3-1370 ppb for conductivity and UV absorption detection, respectlvely. Aromatlc base eluents were applied to the separatlon of calcium and magneslum In seawater and are potentlally sultable for the determlnatlon of aluminum.
INTRODUCTION Detection modes in ion chromatography can be conveniently classified as direct or indirect by comparison of the detector signal occurring during elution of a solute species with the base-line detector signal occurring when no solute is eluted. When the base-line signal is less than that of the solute, the detection mode is direct, whereas indirect detection results when the base-line detector signal exceeds that of the solute.
For example, the most common mode of conductivity detection for anions in ion chromatography is direct (that is, with the use of low-conductance eluents), while indirect UV absorption detection is commonly employed with the aid of strongly absorbing eluent ions. Moreover, use of an aromatic acid anion (such as phthalate) as eluent enables both of these detection modes to be employed simultaneously. Detection modes applicable to nonsuppressed ion chromatography of cations are much more limited, with the general approaches being restricted to indirect conductivity or postcolumn derivatization. It is therefore of interest to determine whether direct conductivity and indirect UV detection using eluents of suitably high absorbance and low conductance can be applied successfully to inorganic cations. Eluent species that have been evaluated for indirect W absorption detection of cations include copper(I1) (1-7), mixtures of copper(I1) and cobalt(I1) (4, 5), cerium(II1) (7), pyridine (2), aniline (for indirect refractive index detection) (8),benzylamine (9),and benzyltrimethylammonium bromide (2). Copper o-sulfobenzoate has been shown to be a suitable eluent for the simultaneous detection of anions and cations using indirect UV absorption (10). Cerium(II1) appears to be the most promising of the inorganic eluent ions, while the organic eluent ions studied to date have given only moderate sensitivity and poor chromatographic selectivity for alkali-metal solute ions. In this paper we provide a comprehensive assessment of the suitability of a range of aromatic bases as eluents for indirect UV absorption and direct conductivity detection of cations in nonsuppressed ion chromatography.
0003-2700/89/0361-1435$01.50/00 1989 American Chemical Society
